The present invention relates to a lithium iron oxide, a method of its synthesis, and a lithium battery including such lithium iron oxide as an electrode active material.
Currently, with the development of portable equipment such as personal computers, handy phones, etc., there has been an increasing demand for a battery as a power source therefor. In particular, lithium batteries have been investigated vigorously as a battery that can ensure a high energy density. This is because lithium has a small atomic weight and can give a high ionization energy.
As the positive electrode active materials used for these lithium batteries, those that can generate a voltage of as high as 4 V, such as Li.sub.x CoO.sub.2 or Li.sub.x NiO.sub.2, have been actively studied recently, in an attempt to increase the electromotive force and energy density of batteries. However, because cobalt or nickel compounds, such as Li.sub.x CoO.sub.2 or Li.sub.x NiO.sub.2, are costly, and outputs of cobalt and nickel are relatively small, such compounds are not optimal materials for practical batteries. Therefore, compounds prepared by substituting Co or Ni in the aforementioned compounds with one of other transition metal elements, particularly iron compounds with a substitution of Co or Ni with iron which has low cost and rich outputs, have been noted to solve the aforementioned problems.
Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2, which are superb electrode active materials with good performance, have a layered rock salt crystal structure (.alpha.-NaFeO.sub.2 structure). Other than Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2, only Li.sub.x VO.sub.2 and Li.sub.x CrO.sub.2 are known to have the layered rock salt crystal structure. There are no known other lithium iron oxides that have an identical layered rock salt crystal structure.
More specifically, a lithium iron oxide with a disordered tetragonal rock salt crystal structure is obtained when a mixture of an iron oxide powder and a lithium compound powder is heated at about 800.degree. C. according to the so-called high temperature synthesis. Whereas, when the mixture is heated at about 400.degree. C. to about 500.degree. C. according to the so-called low temperature synthesis, then a lithium iron oxide with an ordered tetragonal rock salt crystal structure is obtained. However, neither of these lithium iron oxides have proved satisfactory as the electrode active material for practical batteries.